One form of sound-proofing wall used at the present time consists of a frame supported by posts and having a back wall, the frame containing one or more sound-absorbing wall elements which form a sound absorbing layer in spaced relationship to the back wall so that there is a cavity between the back wall of the frame and the sound absorbing layer. This sound absorbing layer faces a source of sound and absorbs some of the incident sound waves, the transmitted residual sound waves impinging on the back wall which reflects some of the sound. The reflected sound passes through the cavity and then back through the layer of sound absorbing wall elements for a second time. The depth of the cavity is therefore chosen according to the frequency of the sound to be absorbed. The sound-absorbing wall elements may be mats of mineral wool protected by an external cover from the weather, from condensation and from mechanical damage. The external cover contains openings of diverse kinds for entry of the sound that is to be absorbed. However, this has the undesirable result that only some of the incident sound actually enters the layer of sound absorbing elements to be absorbed thereby, because part of the sound which impinges upon the surface of the external cover is reflected.